fsmonitor: de-duplicate BUG()s around dirty bits

The index has an fsmonitor_dirty bitmap that records which index entries
are "dirty" based on the response from the FSMonitor. If this bitmap
ever grows larger than the index, then there was an error in how it was
constructed, and it was probably a developer's bug.

There are several BUG() statements that are very similar, so replace
these uses with a simpler assert_index_minimum(). Since there is one
caller that uses a custom 'pos' value instead of the bit_size member, we
cannot simplify it too much. However, the error string is identical in
each, so this simplifies things.

Be sure to add one when checking if a position if valid, since the
minimum is a bound on the expected size.

The end result is that the code is simpler to read while also preserving
these assertions for developers in the FSMonitor space.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsmonitor.c

@@ -13,14 +13,19 @@
 
 struct trace_key trace_fsmonitor = TRACE_KEY_INIT(FSMONITOR);
 
+static void assert_index_minimum(struct index_state *istate, size_t pos)
+{
+	if (pos > istate->cache_nr)
+		BUG("fsmonitor_dirty has more entries than the index (%"PRIuMAX" > %u)",
+		    (uintmax_t)pos, istate->cache_nr);
+}
+
 static void fsmonitor_ewah_callback(size_t pos, void *is)
 {
 	struct index_state *istate = (struct index_state *)is;
 	struct cache_entry *ce;
 
-	if (pos >= istate->cache_nr)
+	assert_index_minimum(istate, pos + 1);
-		BUG("fsmonitor_dirty has more entries than the index (%"PRIuMAX" >= %u)",
-		    (uintmax_t)pos, istate->cache_nr);
 
 	ce = istate->cache[pos];
 	ce->ce_flags &= ~CE_FSMONITOR_VALID;
@@ -82,10 +87,8 @@ int read_fsmonitor_extension(struct index_state *istate, const void *data,
 	}
 	istate->fsmonitor_dirty = fsmonitor_dirty;
 
-	if (!istate->split_index &&
+	if (!istate->split_index)
-	    istate->fsmonitor_dirty->bit_size > istate->cache_nr)
+		assert_index_minimum(istate, istate->fsmonitor_dirty->bit_size);
-		BUG("fsmonitor_dirty has more entries than the index (%"PRIuMAX" > %u)",
-		    (uintmax_t)istate->fsmonitor_dirty->bit_size, istate->cache_nr);
 
 	trace_printf_key(&trace_fsmonitor, "read fsmonitor extension successful");
 	return 0;
@@ -110,10 +113,8 @@ void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate)
 	uint32_t ewah_size = 0;
 	int fixup = 0;
 
-	if (!istate->split_index &&
+	if (!istate->split_index)
-	    istate->fsmonitor_dirty->bit_size > istate->cache_nr)
+		assert_index_minimum(istate, istate->fsmonitor_dirty->bit_size);
-		BUG("fsmonitor_dirty has more entries than the index (%"PRIuMAX" > %u)",
-		    (uintmax_t)istate->fsmonitor_dirty->bit_size, istate->cache_nr);
 
 	put_be32(&hdr_version, INDEX_EXTENSION_VERSION2);
 	strbuf_add(sb, &hdr_version, sizeof(uint32_t));
@@ -335,9 +336,7 @@ void tweak_fsmonitor(struct index_state *istate)
 			}
 
 			/* Mark all previously saved entries as dirty */
-			if (istate->fsmonitor_dirty->bit_size > istate->cache_nr)
+			assert_index_minimum(istate, istate->fsmonitor_dirty->bit_size);
-				BUG("fsmonitor_dirty has more entries than the index (%"PRIuMAX" > %u)",
-				    (uintmax_t)istate->fsmonitor_dirty->bit_size, istate->cache_nr);
 			ewah_each_bit(istate->fsmonitor_dirty, fsmonitor_ewah_callback, istate);
 
 			refresh_fsmonitor(istate);